Gravity flow, sheet flow solar heat collectors having various configurations of water flow, e.g., flat sheet, corrugated channels and the like, and various heat-absorbent media for transferring heat to the fluid, e.g., metal, dark-colored plastic and the like, have been suggested in the prior art. However, despite certain advantages which such collectors have over the expensive tubular collectors, in such terms as reduced leakage and lower construction costs, they have not been practically feasible for use in the temperate to frigid latitudes.
In such latitudes, for optimum radiation absorption, the collector must be tilted to a substantial angle relative to the horizontal so as to be substantially perpendicular to the rays of the sun at solar noon. At such angles, gravitational effects on the water, which is used because of its high specific heat, cause such high flow velocities that exposure time per pass through the collector panel (typically 4 by 8 feet) and, thereby, the amount of heat absorbed is exceedingly low. The large number of passes per unit of time required to absorb an adequate amount of heat energy results in power input requirements which are very high relative to the energy output, rendering the system excessively inefficient in terms of costly energy input. The high flow rates also result in excessively noisy systems which wear out relatively quickly. Additionally, because of the low exposure time per pass, the difference between panel input and output fluid temperatures (.DELTA.T) requires exceedingly .DELTA.T-sensitive, costly switch devices for shutting off operation of the collector during night hours or adverse cloud conditions.
Minardi et al, U.S. Pat. No. 3,939,819, and Minardi et al, Performance of a "Black " Liquid Flat-Plate Solar Collector, Solar Energy, Volume 17, pages 179-183, disclose the use of "black" liquids as the direct radiant heat energy absorbent, thus eliminating the requirement for an intermediate heat absorbent, e.g., metal or dark-colored plastic (coated or per se), with resultant reduction in over-all system temperature and reduced panel weight. The panel system disclosed is a tubular system employing transparent tubing for fluid passage. The fluid is pumped through the tubing. Passing reference is made to use of the "black" fluid medium in sheet-flow, and FIG. 4 in U.S. Pat. No. 3,939,819 shows a gravity sheet flow system tilted at an angle of 43.degree.. The references are devoid of any reference to viscosity of their fluids or its effect on sheet flow. It should be noted that increased fluid viscosity is detrimental to non-gravity flow devices. The highest viscosity fluid disclosed by the Minardi et al references consists of 3 parts of ethylene glycol, which is employed as an antifreeze, admixed with 1 part of a water mixture made by dispersing 91 parts of Acheson's Aquadag (a dispersion of colloidal graphite in water). Viscosity of this mixture, as measured at 120.degree. F., equals 7 centipoise.
Goddard U.S. Pat. No. 1,951,403 discloses a tubular collector wherein the heat-absorbent medium is a reservoir of an oil suspension containing carbon. The tubes, which carry a different heat-absorbent liquid presumably the conventional water, are embedded in the oil/C reservoir. Goddard does not indicate the reason for selecting his particular reservoir medium and is silent concerning viscosity. Although the oil suspension may have the higher viscosities required by the present invention, viscosity plays no role in the reference collector system.